A flash memory is a non-volatile storage device that can be electrically erased and reprogrammed. Flash memories are commonly used in memory cards, USB flash drives and solid state drives for general storage and transfer of data between computers and other digital products. Flash memory devices typically store information in an array of memory cells made using floating gate transistors.
A floating gate transistor is a field effect transistor having a structure similar to a conventional metal oxide semiconductor field effect transistor (“MOSFET.”) Floating gate MOSFETs are distinguished from conventional MOSFETs because the floating gate transistor includes two gates instead of one. In addition to an upper control gate, a floating gate transistor includes an additional floating gate between the control gate and above the transistor channel, but completely electrically isolated by an insulating layer such as an oxide layer that completely surrounds the floating gate.
This electrically isolated floating gate creates a floating node in direct current “DC” operation with a number of inputs or secondary gates such as the control gate, formed above the floating gate and electrically isolated from it. These secondary gates or inputs are capacitively connected to the floating gate. The coupling ratio, Cr, between the control gate or other secondary gate, and the floating gate is determined by the capacitances of the dielectric between the floating gate and the control gate and also the dielectric between the floating gate and the source line. The dielectric between the floating gate and the source line is commonly the gate oxide or other dielectric between the floating gate and the substrate in the area in which the floating gate overlaps the source line diffusion region. Providing a high coupling ratio and minimizing floating gate cell leakage are useful to programming efficiency of the flash cell and this is especially important as the cell size shrinks.